1. Field of the Invention
The present invention relates to a photoelectric conversion element and provides a photoelectric conversion element with high S/N and high response speed by specifying the materials and structures.
2. Description of Related Art
Conventional visible light sensors in general are a device fabricated by forming a photoelectric conversion site through, for example, formation of PN junction in a semiconductor such as Si. As for the solid-state imaging device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and a signal generated by photoelectric conversion in each pixel is charge-transferred and read out according to a CCD or CMOS format. The method for realizing a color solid-state imaging device is generally fabrication of a structure where on the light incident surface side of the flat image-receiving device, a color filter transmitting only light at a specific wavelength is disposed for color separation. In particular, a single-plate sensor in which color filters transmitting blue light, green light and red light, respectively, are regularly disposed on each of two-dimensionally arrayed pixels is well known as a system widely used at present in a digital camera and the like.
In this system, since the color filter transmits only light at a limited wavelength, untransmitted light is not utilized and the light utilization efficiency is bad. Also, in recent years, amid the advance in fabrication of a multipixel device, the pixel size and in turn, the area of a photodiode part become small and this brings about problems of reduction in the aperture ratio and reduction in the light collection efficiency.
In order to solve these problems, there may be considered a system where photoelectric conversion parts capable of detecting light at different wavelengths are stacked in a longitudinal direction. As regards such a system, for example, U.S. Pat. No. 5,965,875 discloses a sensor utilizing wavelength dependency of the absorption coefficient of Si, where a vertical stacked structure is formed and the colors are separated by the difference in the depth, and JP-A-2003-332551 discloses a sensor by a stacked structure using an organic photoelectric conversion layer. However, the system by the difference in the depth direction of Si is originally disadvantageous in that the color separation is poor, because the absorption range is overlapped among respective portions and the spectroscopic property is bad. As for other methods to solve the problems, a structure where a photoelectric conversion layer by amorphous silicon or an organic photoelectric conversion layer is formed on a signal reading substrate is known as a technique for raising the aperture ratio.
Heretofore, several examples have been known for a photoelectric conversion element, an imaging device, a photosensor and a solar cell each using an organic photoelectric conversion layer. A high photoelectric conversion efficiency and a low dark current are a problem in particular, and as to the improvement method in this respect, there are disclosed, for example, introduction of a pn-junction or introduction of a bulk-heterostructure for the former and introduction of a blocking layer for the latter.
In an attempt to raise the photoelectric conversion efficiency by the introduction of pn-junction or bulk-heterostructure, an increase in the dark current often becomes a problem. Also, the degree of improvement in the photoelectric conversion efficiency differs depending on the combination of materials and in some cases, the ratio of light-signal amount/dark time noise does not increase from before introduction of the structure above. In the case of employing the method above, what materials are combined is important and in particular, when reduction in the dark time noise is intended, this is difficult to achieve by already reported combinations of materials.
Furthermore, the kind of the material used and the layer structure are not only one of main factors for the photoelectric conversion efficiency (exciton dissociation efficiency, charge transport property) and dark current (e.g., amount of dark time carrier) but also a governing factor for the signal responsivity, though this is scarcely mentioned in past reports. In use as a solid-state imaging device, all of high photoelectric conversion efficiency, low dark current and high response speed need to be satisfied, but there has not been specifically disclosed what an organic photoelectric conversion material or a device structure satisfies this requirement.
A photoelectric conversion layer containing fullerenes is described in JP-A-2007-123707, but only by fullerenes, it is impossible to satisfy all of the above-described high photoelectric conversion efficiency, low dark current and high response speed. Also, JP-A-2002-076391 describes a solar cell using a bulk-heterostructure layer including a plurality of organic semiconductors, with at least one organic semiconductor being a crystal grain, where, however, disclosure on dark current and high-speed response is not found and application or the like to a photoelectric conversion element for imaging devices is neither described nor suggested.